1. Field of the Invention
The present invention relates to leakage diagnosis in an evaporated gas purge system for diagnosing existence or absence of leakage (pressure leakage) of an evaporated gas purge system for purging (discharging) evaporated gas caused by evaporation of fuel in a fuel tank to an intake pipe of an internal combustion engine.
2. Description of Related Art
Conventionally, in an evaporated gas purge system, in order to prevent evaporated gas generated from inside of a fuel tank from leaking out to the atmosphere, the evaporated gas is adsorbed in a canister via an evaporated gas passage at the inside of the fuel tank, a purge control valve is installed midway of a purge passage for purging evaporated gas adsorbed in the canister to an intake pipe of an internal combustion engine and opening and closing of the purge control valve is controlled in accordance with an operating state of the internal combustion engine by which, the flow rate of the evaporated gas purged from the canister to the intake pipe is controlled. In order to prevent extensive leakage of evaporated gas from the evaporated gas purge system to atmosphere, the leakage of the evaporated gas needs to be detected at an early stage.
Hence, as disclosed in, for example, Japanese Patent Application Laid-Open No. Hei-5-125997, a purge system, in which existence or absence of leakage is diagnosed based on pressure in the purge system or an amount of pressure change after atmosphere or negative pressure of an intake pipe is introduced and hermetically sealed in the purge system including a fuel tank and a canister, is known.
The leakage diagnosing operation may be carried out during idling of an engine and therefore, the filler cap of the fuel tank may be opened for supplying fuel or the like when leakage diagnosis is being carried out by which the purge system may be opened to the atmosphere during leakage diagnosing operation. That is, when fuel supply is carried out while running an engine, the filler cap may be opened (filler opening may be opened) during leakage diagnosing operation.
When the filler cap is opened and the purge system is opened to the atmosphere during leakage diagnosing operation, the purge system is erroneously diagnosed as having leakage even when the system is normal from the start since the system is brought into a state the same as that when a large hole is opened in the purge system.
In order to prevent such an erroneous diagnosis, as disclosed in Japanese Patent Application Laid-Open No. Hei-9-137756, there is a known system in which a fuel temperature sensor is installed to a fuel tank, existence or absence of lowering of fuel temperature is monitored and when the fuel temperature is lowered, it is determined that fuel is being supplied and leakage diagnosis is prohibited.
However, according to this related art, the fuel temperature sensor needs to be installed at the fuel tank.
Further, when the temperature difference between temperature of fuel in the fuel tank and temperature of the supplied fuel is small, it cannot be determined that fuel is being supplied. Furthermore, if the leakage diagnosis is finished after the filler cap is opened and before the fuel supply is started, it is not determined that fuel is being supplied, and accordingly, the system is erroneously diagnosed without canceling the diagnosed result even when it is diagnosed that the leakage is caused by opening the filler cap. After all, according to the determination of fuel supply by fuel temperature, an erroneous diagnosis caused by opening the filler cap may not be prevented, and reliability of leakage diagnosis cannot be improved sufficiently.